The term "coal" has been used to describe generically a wide variety of naturally-occurring materials which are, in fact, sedimentary rocks of high, but variable, organic content also containing inorganic mineral constituents in varying amounts. The inorganic mineral content of coal can vary widely in composition. In addition, the extent of "coalification" which has occurred in different coals varies significantly and has led to the useful practice of referring to coals by "rank". In accordance with this practice Class I coals are the anthracites; Class II coals are the bituminous; Class III coals are the sub-bituminous and Class IV coals are the lignites. Generally, the lower-ranking coals will have more ash and moisture and lower heat value that the higher ranking coals.
As shown in the book Low Rank Coal Technology by Gronhovd, et al, Noyes Data Corporation, 1982, the ash compositions of different-ranking coals are different. Thus, the following table occurs at page 3 of the aforementioned book:
______________________________________ Average Ash Compositions From U.S. Coals of Different Ranks (Weight Percent of SO.sub.3 - Free Ash) Lignite Subbituminous Bituminous ______________________________________ SiO.sub.2 24.9 39.4 48.1 Acidic Al.sub.2 O.sub.3 14.1 21.1 24.9 Components Fe.sub.2 O.sub.3 11.5 10.1 14.9 TiO.sub.2 0.5 0.8 1.1 P.sub.2 O.sub.5 0.4 0.4 -- CaO 31.2 20.1 6.6 Alkali MgO 8.7 5.6 1.7 Components Na.sub.2 O 8.2 2.1 1.2 K.sub.2 O 0.5 0.3 1.5 ______________________________________
The inorganic or ash constituents of coals are finely disseminated through the organic or carbonaceous constituents so that commonly-used mineral beneficiation techniques are of only limited value in affecting separation. Thus, jigging, dense media separation, cycloning or froth flotation have been used with indifferent results. The ash constituents thus accompany the organic constituents of coal when the coal is crushed or ground and burned, as for example, in a power plant boiler. No heat value is contributed by the ash and combustion gases become laden with the incombustible inorganic material. The ash constituents of some coals are of such composition that they form molten slags which deposit upon boiler tubes and other parts of the combustion apparatus. Such slag deposits interfere with heat transfer across boiler tube surfaces and in some cases are found to corrode boiler tubes and other parts of the combustion apparatus leading to premature failure thereof. Solid ash or slag particles are also destructive and erode boiler tubes and other parts of boilers and other apparatus used for burning coal whether such parts are of metal or refractory. Disposal of the unburned ash is also a problem, whether the ash remains on furnace grates, is captured as fly ash, or otherwise. The melting points of ash from various coals varies widely and in the case of some coals is so low that the ash is removed from the combustion apparatus as a molten slag.
Coal is cheap and plentiful. Many coals approach petroleum oils in terms of heat value per unit weight upon combustion. Direct use of powdered coal as a water slurry or otherwise in heat engines such as gas turbines or diesel engines has been postulated but the destructive effects of the ash constituent of coals due to erosion or corrosion of metal parts within the engines has created a climate of despair on the question of whether such direct use would ever be practical.
It has also been postulated that dispersion of pulverized coal in petroleum-derived fuel oil would provide a substantial increase in heat value of the mixture per unit volume as compared to fuel oil, thereby permitting increased range of steam-powered ships as well as other advantages. However, this potential has not been realized on an extensive scale due to the ash content of coal.
It has been suggested in U.S. Pat. No. 3,993,455 that caustic leaching followed by acid washing be used to reduce the ash content of coal to acceptable levels. However, it is very difficult to remove residual caustic from caustic-treated coal and sodium potassium in coal is itself harmful to mechanical elements of coal combustion equipment.
Despite the problems which have arisen in attempting to employ coal slurry as fuel for gas turbines and the like, economic factors promoting such use are powerful and it has been postulated that, in view of what is known from fuel oil experiences about the harmful effects of ash, alkali metals and vanadium upon gas turbines, the maximum permissible amounts in coal-slurry gas turbine fuel will be 0.85% ash, by weight, 150 ppm alkali metals (Na,K) and 4 ppm vanadium.
A difficult technical problem thus remains in the treatment of coal to provide such low contents of undesired material therein.